1. Technical Field
This invention relates generally to directed energy beam steering systems and, more particularly, to an optical beam steering system having microlens arrays and a phase compensating lens for steering high power optical beams.
2. Discussion
The ability to achieve fast retargeting over a wide field of view has become an important issue for strategic and tactical optical targeting applications, especially those involving moderate to high power laser systems. For military purposes, retargeting speeds are an important factor in determining the effectiveness of a laser system in targeting and killing a sufficient number of targets within a wide field of view over a limited time frame. In addition, fast retargeting is desirable for laser systems commonly employed for other space-related applications such as laser communications applications which typically require beam agility with small size and low weight components. In order to accomplish fast retargeting, it is generally necessary to employ a beam steering system which redirects an optical beam along a desired path.
In the past, many of the early conventional beam steering systems commonly required a rather complicated rotating mechanism to move a large focusing mirror around a rotation region. According to such conventional approaches, the steering system response is generally inversely scaled to the size of the focusing mirror. However, high power optical beams generally require a relatively large mirror to effectively redirect the energy beam. The requirement of a large mirror leads to a slow steering response, especially for wide-angle steering applications in which more mirror movement is generally required. Accordingly, in order to obtain high power wide-angle cost-effective beam steering, there existed a need for a beam-directing system which could provide improved response time and simplified control mechanisms.
Presently, there exists a microlens beam steering system which includes the translation of two microlens arrays configured as a telescope array. Such a system is described in a technical paper by Flood et al, entitled "Continuous Wide Angle Beam Steering Using Translation of Binary Microlens Arrays and a Liquid-Crystal Phased Array", published in SPIE Proceeding Volume 1211, pages 296-304, (1990). This technical paper is hereby incorporated by reference. Using this type of microlens beam steering technique, an optical beam is separated into a plurality of beam columns, each of which passes through a microlens in the microlens array. Translational movement of the microlens arrays provides the ability to steer the plurality of beam columns collectively. To complete the beam steering, the steered beams are then recombined to provide a redirected far-field beam. According to this approach, wide-angle beam steering is achieved with a relatively small translational movement which generally requires movement of only a fraction of the size of the microlens.
While the above-described microlens array beam steering approach provides a number of advantages over the prior rotating mirror beam-directing systems, a number of drawbacks still exist. First, the plurality of steered beam columns generally are not uniformly in phase with one another, with the exception of certain discrete steering angles. As a consequence, the resulting far-field beam intensity will generally be rather low, unless the beam columns can be uniformly brought back into phase with one another. A second drawback associated with some of the prior approaches involves the existence of interference which is commonly present among adjacent beam columns and steering lenses.
The above-identified article further discloses a method using a liquid-crystal phase-modulated array for providing phase compensation among the beam columns so as to eliminate some of the discrete steering angle limitations. However, the liquid-crystal arrays usually require individual transistors located in each subaperture to control the voltage applied thereto. This requirement unduly complicates the control system. In addition, liquid crystals are generally not desirable materials for use with moderate to high power laser beam applications. That is because liquid crystal is generally known to have a rather low optical damage threshold and therefore is easily susceptible to damage such as that caused by burning. Furthermore, liquid crystal is currently a rather costly material.
It is therefore desirable to provide for an improved beam steering system which does not suffer from the above described drawbacks commonly found with conventional approaches. In particular, it is desirable to provide for a laser beam steering system having a microlens array which compensates for phase differences between laser beam columns so as to provide phase compensation therebetween. In addition, it is desirable to provide for such a beam steering system which has a wide-angle and high-speed response and a more simplified control scheme.